The primary consideration in electroplating aluminum or aluminum alloys is the presence of an oxide film on the aluminum surface which prevents adequate and uniform adhesion of plating deposits. The oxide film is sometimes considered a natural film because it is consistently present on aluminum when exposed to the atmosphere or to any medium that contains oxygen. Even though the film be removed, it forms extremely rapidly upon re-exposure to oxygen. Due to aluminum's high affinity for oxygen and to its position in the electromotive series, being anodic to all common metals except zinc and cadmium, the commercial application of electroplated aluminum alloys has been severely limited.
Historical efforts to achieve good adhesion of electroplating on aluminum, has included the use of a direct plated zinc layer as early as 1931, but more recent efforts have included the use of an immersion zincate treatment, and a tin/bronze pre-plating.
A number of pre-plating treatments or underlayment systems have been employed by the prior art with the hope of solving the adherency problem. Those which have achieved some degree of commercial use fall into basically three categories: (a) the use of zinc because zinc is anodic to aluminum and can be deposited by immersion, (b) a tin/bronze underlayment, tin being anodic to zinc, or (c) a phosphoric acid anodized underlayer. Zinc, as a heavy plated underlayment, has been reportedly used as early as 1931. But more recent efforts have employed zinc by an immersion technique commonly referred to as a zincate treatment. Unfortunately, the immersion technique is more an art than a science because the actual control parameters of the process are not well understood and undesirable variances appear. The extremely high zinc content of the underlayment is readily attacked and dissolved in subsequent acid dips or plates necessary to electroplating nickel if not protected by additional barrier elements or double thickness. Most importantly, the presence of the zinc in contact with the aluminum, sets up an electrolytic cell which promotes lateral corrosion along the zinc layer, the zinc being sacrificial, after a slight scratch or fracture occurs through the outer plated system.
The tin/bronze pretreatment employs an electrolytic or immersion tin deposit to delay the oxidation of the aluminum. In order to avoid the generation of blisters within the underlayment, the transfer time of the aluminum parts between the tin bath and the bronze bath is unfortunately limited to 12 seconds or less. Almost all available production equipment is not capable of consistently carrying out such a rapid transfer time and therefore the use of the tin/bronze technique on most plating plants does not render successful plating results.
Phosphoric acid anodizing generates a very thin film of aluminum oxide which is tightly adhered to the aluminum substrate, and in turn is employed to bond to the outer metallic coatings. However, the oxide film is extremely brittle (equivalent to the brittleness of glass) and will fracture with slight deformation. Moreover, the oxide film as the initial deposit, is technically a mere coating; consequently the adhesion of the subsequent metallic overlayers to the aluminum substrate becomes a mechanical attachment rather than a molecular bond as is normal in electroplating. The net result is a much poorer attachment of the plating system.